Field of the Invention
The present invention relates to a magnetic recording medium and a magnetic coating composition for a magnetic recording medium.
Discussion of the Background
Particulate magnetic recording media (also referred to as simply “magnetic recording media” hereinafter) are normally fabricated by coating a magnetic coating composition comprising ferromagnetic powder and binder on a nonmagnetic support, either directly or indirectly via another layer such as a nonmagnetic layer, and conducting a curing treatment such as heating or irradiation with light as needed to form a magnetic layer.
Conventionally, binders have played an important role in enhancing the dispersion of ferromagnetic powder and the durability of the magnetic layer in particulate magnetic recording media. As a result, various research has been conducted on binders (for an example, see Japanese Unexamined Patent Publication (KOKAI) No. 2004-67941, which is expressly incorporated herein by reference in its entirety).
As stated in Japanese Unexamined Patent Publication (KOKAI) No. 2004-67941, polar groups such as sulfonate groups have been incorporated into binder to enhance the dispersion of ferromagnetic powder. The introduction of polar groups into binder has been conducted to increase dispersion by causing the binder to effectively adsorb to the surface of ferromagnetic powder. However, as is described in paragraph 0026 of Japanese Unexamined Patent Publication (KOKAI) No. 2004-67941, the introduction of an excessive quantity of polar groups tends to decrease the dispersion of ferromagnetic powder. Accordingly, it has proven difficult to achieve an adequate improvement in the dispersion of ferromagnetic powder by introducing polar groups into binder.
The use of resins with good mechanical properties as binder in the magnetic layer has been examined to enhance the durability of the magnetic layer. In this regard, Japanese Unexamined Patent Publication (KOKAI) No. 2004-67941 proposes using a prescribed copolymerization component such as an aromatic polyisocyanate to increase the concentration of urethane groups in order to enhance the mechanical properties of the polyurethane resin that is used as binder in the magnetic layer. As stated in paragraph 0025 of Japanese Unexamined Patent Publication (KOKAI) No. 2004-67941, the higher the concentration of urethane groups of the resin that is used as binder, the better the mechanical properties that can be achieved in the resin. However, solubility decreases. As a result, dispersion of the ferromagnetic powder tends to decrease. Thus, Japanese Unexamined Patent Publication (KOKAI) No. 2004-67941 states in paragraph 0025 that the concentration of urethane groups is to be in a range that permits good dispersion of ferromagnetic powder.
Even greater durability has been required of the magnetic layer in recent years. Reasons for this include the fact that the market has demanded ever higher properties and the size of ferromagnetic powder has been decreasing. For example, properties required by the market in recent years have included high durability in continuous running over longer periods with higher reliability than in the past. Since the magnetic force per bit has weakened accompanying the reduction in particle size of ferromagnetic powder, the distance between the reproduction head and the surface of the magnetic recording medium (magnetic layer) has tended to decrease in order to read information from such bits. Thus, the frequency of contact between the reproduction head and the surface of the magnetic recording medium (magnetic layer) has increased in recent years. Accordingly, magnetic recording media have come to be used in states where the surface of the magnetic layer is more prone to being scratched than in the past.
Accordingly, it is conceivable to enhance the mechanical properties of the resin that is employed as binder in the magnetic layer, as has been examined in the past, to increase the durability of the magnetic layer. However, as set forth above, the better the mechanical properties of the binder are rendered to enhance the durability of the magnetic layer, the poorer the dispersion property of the ferromagnetic powder tends to be. That is, achieving both the greater durability of the magnetic layer that has been demanded in recent years along with better dispersion of ferromagnetic powder will be difficult with binders such as those that have been conventionally employed.